The objective of this program is to understand the mechanism(s) of cellular changes fundamental to neoplastic transformation. Retroviral onc gene(s) or their mitogenic onc gene product(s) may subvert normal growth regulatory mechanisms and cause neoplastic transformation in culture. Nonneoplastic mammalian cells in culture have specific hormone and growth factor requirements for initiation of DNA synthesis and mitosis, and rigorous analysis is possible only in the absence of serum mitogens. A serum-free model system which supports proliferation and maintenance of nonneoplastic NIH/3T3 cells for up to three weeks was developed. Changes in insulin, epidermal growth factor and basic fibroblast growth factor requirements were assessed with a known prototype onc gene which codes for a potent mitogen (v-sis) or a defective cell membrane receptor (v- erbB), or is involved in intracellular transduction of mitotic stimuli. In this biologic assay system, nonneoplastic NIH/3T3 cells remain sensitive to the constraints or normal growth regulation when tested as single cells (clonal growth) or as proliferating population at higher density. Introduction of an onc gene causes unique changes in growth factor requirements. Cells transfected with v-sis and v-erbB generally are less dependent upon a set of competence and progression factors than cells transfected with v-ras, which, like progenitor NIH/3T3 cells, need at least two growth factors for survival and growth. Knowledge of the impact these genes have on normal cells will lead to strategies for counteracting their tumorigenic potential.